This invention pertains to coalescence enhanced gravity separation of fine sized iron-based catalyst from catalyst/waxy reactor effluent slurries in Fischer-Tropsch synthesis processes. It pertains particularly to such enhanced separation and gravity settling of the fine catalysts by utilizing contact with a coalescence enhancing treating solution to facilitate the catalyst separation and rapid settling from the catalyst/wax reactor effluent slurries.
Iron-based catalysts selected for use in Fischer-Tropsch (F-T) process slurry-bed reactors have initial particle size up to about 100 microns, but undesirably break down very easily under the reaction conditions into very fine particles having sizes in the 1-2 micron and even sub-micron ranges. These extremely fine catalyst particles make desired separation of such catalyst fines from the F-T process waxy liquid products including paraffin wax very difficult. Such catalyst separation problems result in unacceptable product quality and undesired loss of catalyst, and have thus far prevented commercialization of iron-catalyzed slurry-phase Fischer-Tropsch synthesis processes.
Various catalyst/wax separation techniques have been studied, including conventional gravity settling, filtration (both internal and external), centrifugation including hydrocloning, solvent extraction and magnetic separation; however, they have not been commercially successful. For example, U.S. Pat. No. 4,605,678 to Brennan et al discloses separating Fischer-Tropsch (F-T) process catalyst fines from the waxy product by passing it through a high gradient magnetic field, so that the catalyst is retained by a magnetized filter element. U.S. Pat. No. 5,520,890 to Lorentzen discloses separating slurry phase containing catalyst from liquid product utilizing vertical filtering tubes in a reactor. U.S. Pat. No. 5,827,903 to White et al discloses a method for separating catalyst fines from F-T synthesis wax products by dense gas and/or liquid extraction steps. U.S. Pat. No. 6,068,760 to Benham et al discloses a method for separating catalyst particles from F-T process wax product by utilizing a specially shaped dynamic settler device. Also, U.S. Pat. No. 6,096,789 to Clerici discloses a F-T synthesis process for producing hydrocarbons in which fine catalyst particles are separated from a liquid slurry utilizing dual staged hydroclone units. Sasol in South Africa is apparently the only producer that has disclosed successful use of an internal filter having a proprietary complicated design for their slurry-bed iron catalyst F-T reactors. Thus, the iron catalyst/wax separation techniques investigated for F-T synthesis processes are either not able or efficient enough to meet the requirements for producing clean hydrocarbon liquid products, or are too complicated and expensive to be economically justified. However, a coalescence enhanced settling technique that facilitates separation of the iron-based catalyst fines from the F-T synthesis process catalyst/wax reactor effluent slurry in an effective and economic manner is very desirable, and is now being provided.
This invention provides an improved method for separation and gravity settling of fine iron-based catalyst in catalyst/wax reactor effluent slurries in Fischer-Tropsch (F-T) synthesis processes for producing substantially clean hydrocarbon liquid/wax products by utilizing a coalescence enhanced separation and gravity setting procedure for the catalyst fines. The invention utilizes a special aqueous treating solution for contacting or mixing with a catalyst/liquid/wax slurry withdrawn from a F-T process reactor, and induces enhanced coalescing of the catalyst fines into larger clusters or globules, thereby enabling their effective separation from the catalyst/wax slurry medium by rapid gravitational settling.
Suitable aqueous treating solutions for this invention include two or more chemical agents selected for providing reduced surface tension in the catalyst/liquid/wax slurry, along with catalyst agglomerating and coalescing steps for the fine catalyst as utilized for the method of this invention. Such treating solutions include an interfacial or surface tension reducing component, an agglomerating or agglutinating component, and a coalescing component for the catalyst fines. Although the catalyst sedimentation mechanism is not entirely understood, it is believed that the interfacial tension reducing component lowers surface tension of water so that it wets the catalyst fines which are held in an unstable oil/water emulsion formed by the agglomerating and/or coalescing components. Surface tension reducing agents include alcohols such as ethanol and 2-propanol which help reduce interfacial tension, and components such as EDTA, methyl cellulose and methyl sulfoxide facilitate agglomeration and coalescence of the fine catalyst particles into larger clusters or globules, and thereby facilitate rapid settling of the catalyst particles in the liquid/wax slurries.
The coalescence enhanced separation and settling method or procedure of this invention requires bringing the aqueous treating solution into intimate contact with the catalyst/liquid/wax slurry by a suitable mixing step. Such mixing step can be, provided by hydraulic, mechanical, magnetic, or ultrasonic mixing means located either upstream from or within an upper portion of a pressurizable settling container or tank so as to form the catalyst clusters or globules, followed by rapid gravitational settling of the catalyst clusters/globules formed. In the settling container, the catalyst/wax slurry and treating solution are maintained in a liquid form or state to facilitate such intimate contact and catalyst settling. Following such mixing step, the separation and settling of the fine catalyst for most catalyst/wax slurries usually occurs in the container relatively rapidly in less than about 10 minutes, and preferably in less than 6 minutes, so as to provide at least about 90% separation and settling of the catalyst, and preferably to provide a substantially clear hydrocarbon liquid/wax product. However, for catalyst/wax slurries containing very high molecular weight hard wax products, such separation and settling of the coalesced catalyst may require up to 15 minutes.
Proper chemical composition for the treating solution is essential for successful separation and gravity settling of the catalyst fines from the liquid/wax slurry product for this invention. This proprietary treating solution preferably includes three component agents in aqueous solution as follows:
Agent Axe2x80x94an surface tension reducing agent selected for reducing interfacial tension between the catalyst fine particles and hydrocarbon liquid/wax product, and which is usually an alcohol, from C1OH to C10OH monoalcohols, and di-alcohols, e.g., ethylene glycol, and such. Concentration in the treating solution should be 1-80 vol. %.
Agent Bxe2x80x94an agglomerating or agglutinating agent, such as acrylic acid, methyl cellulose, polyacrylic acid and such. Concentration in the treating solution should be 0.001-10 vol. %
Agent Cxe2x80x94A coalescing agent for fine catalyst particles, such as glycerin, methyl sulfoxide, 2, 3-dimercapto-1-propanesulfonnic acid (DMPS), meso-2, 3-dimerecaptosuccinic acid (DMSA), ethylenediamine-tetraacetic acid (EDTA), diethylenetriamine pentaacetic acid (DTPA), desferrioxamine, dithiocarbarnate (DTC), penicillamine, and
Deinonized water provides the remainder of the treating solution.
This method for effective catalyst/wax separation by coalescence enhanced gravity settling of the fine catalyst from the liquid/wax slurry product and the treating solution can be performed under the following ranges of operating or treating conditions:
Catalyst/wax slurry temperature between 10xc2x0 C. and 250xc2x0 C., with the temperature being at least 10xc2x0 C. above the melting point and at least 10xc2x0 C. below the boiling point of the treating solution, and preferably at 20-200xc2x0 C. slurry temperature.
Settling tank pressure for treated catalyst/wax slurry sufficient to maintain the treating solution in liquid state, and usually from atmospheric up to about 500 psig, and preferably 5-400 psig tank pressure.
Volume ratio of treating solution to catalyst/liquid/wax slurry in 1-5:1 range, and preferably is 1.5-4.5:1.
Whenever the catalyst/wax slurry to. be treated has a wax melting temperature above about 50xc2x0 C., or less than about 10xc2x0 C. below the treating temperature, diluting the catalyst/wax slurry with a light hydrocarbon solvent such as heptane, hexane, or light naphtha may be necessary to facilitate catalyst separation and rapid settling of the catalyst fines following the catalyst/wax treatment step. Following the mixing step in which the catalyst/liquid/wax slurry is contacted by the treating solution, substantially all of the catalyst fines are usually separated from the hydrocarbon liquid/wax slurry product and the treating solution within less than about 10 minutes, and preferably less than 6 minutes, so that the hydrocarbon liquid/wax product becomes essentially visually clear.
It is a significant advantage of this invention that it provides a method and procedure for efficient separation and rapid settling of essentially all the catalyst fines contained in the liquid/wax product slurry from a F-T synthesis process. This separation method thereby enables successfully producing clean high quality hydrocarbon liquid/wax products, recovering the treating solution for reuse, and recycling a portion of the separated catalyst fines back to the F-T reactor if desired for reuse in an efficient manner.